1. Field of the Invention
The present invention relates to a drive train for a motor vehicle which is driven by an internal combustion engine, having:                an individual friction clutch which has an input element and an output element, wherein the input element is to be connected to a crankshaft of the internal combustion engine;        a step-by-step variable speed transmission which has an input shaft, a multiplicity of wheel sets corresponding to a multiplicity of gear speeds, and an output shaft, wherein the input shaft is connected to the output element of the friction clutch; and        an electric machine which can be connected via a clutch to the output shaft of the step-by-step variable speed transmission in order to be able to apply traction force to the output shaft of the step-by-step variable speed transmission when the friction clutch is opened or closed, and which can be connected as a starter generator to the crankshaft in order to start the internal combustion engine or be driven by the internal combustion engine.        
The present invention also relates to a method for starting an internal combustion engine of a motor vehicle and for initiating the starting up of the motor vehicle.
Finally, the present invention relates to a method for generating electric current while the motor vehicle is stationary by means of an electric machine which is driven by an internal combustion engine of a vehicle.
2. Description of Related Art
It has been known for a long time to integrate an electric machine into the drive train of a motor vehicle which is driven by an internal combustion engine. In this context, the functions of starting the internal combustion engine and generating current for the on-board electric system are conventionally implemented by means of two separate electric machines, the starter and the dynamo (generator), respectively.
For on-board electric systems of the motor vehicle with relatively high rated voltages (for example 42 volts on-board electric system) the possibility of providing an electric machine as a starter generator between the internal combustion engine and friction clutch is generally also considered. When the friction clutch is opened, the electric machine can be operated as a motor for starting the internal combustion engine. When the internal combustion engine is running, the electric machine can be used as a generator for feeding the on-board electric system. Cf. Bosch Kraftfahrtechnisches Taschenbuch [Bosch Motor Vehicle Handbook], 24th edition, page 932.
An electric machine in the drive train can also be used for other functions.
For example it is known to use the electric machine as a “booster” when there is a desire by the driver for high acceleration so that the performance of the internal combustion engine and the performance of the electric machine are added at the output.
In addition it is possible to convert the centrifugal energy (inertia) remaining when the vehicle is braked or when the engine is switched off into electrical energy (recuperation). The energy which is acquired in this way can be used to support the starting up process.
In addition it is known to assign an electric machine to the output side of the transmission so that when changing the gear speed the electric machine can be used to apply traction force to the output shaft of the transmission during phases when the friction clutch is opened. This brings about boosting of the traction force, which is applied in particular in what is referred to as automated manual shift transmissions.
Active synchronization is another function of an electric machine in the drive train. In such a case the electric machine is connected to at least one transmission shaft of a step-by-step variable speed transmission and is used to brake or accelerate the shaft for the purpose of synchronization with the clutch of a gear speed which is to be engaged. In this embodiment, it is possible to dispense with mechanical synchronization at least for some clutches so that the clutches can be embodied as simple claw clutches or the like.
Finally it is also known that switching off the internal combustion engine in the stationary state (for example at traffic lights) and not restarting until there is a desire to drive off offers considerable potential for saving fuel.
Consequently, a function which is desired for integrated electric machines is the supporting of such direct starting after the internal combustion engine has been switched off.
The document “Mögliche Anordnung des Startergenerators im Antriebsstrang [Possible arrangement of the starter generator in the drive train]” by Dr.-Ing. Wolfgang Reik, Tagungsband zur Fachtagung “E-Maschine im Antriebsstrang” [Conference papers at the specialist conference “The electric machine in the drive train”], Apr. 9, 1999, discloses an overview of the possible arrangements of an electric machine in the drive train and the advantages and disadvantages which are associated with the respective arrangement.
For example, a connection of the electric machine to the crankshaft of the internal combustion engine has the advantage that a direct start is possible, but the traction force cannot be boosted.
An arrangement between the friction clutch and transmission has the advantage that recuperation is possible but both a direct start and boosting of the traction force are impossible.
An electric machine which is arranged in the transmission permits both boosting of the traction force and recuperation but frequently does not allow the electric machine to be operated as a starter generator since generally not only the friction clutch but also at least one of the clutches of the step-by-step variable speed transmission have to be closed in order to drive the internal combustion engine.
“Integration automatisierter Schaltgetriebe mit E-Maschine [Integration of automated transmissions with an electric machine]” by Dr. Robert Fischer et al., Tagungsband zur Fachtagung “E-Maschine im Antriebsstrang [Conference papers for the specialist conference “The electric machine in the drive train”], Apr. 9, 1999, discloses that an electric machine is to be arranged in the drive train in such a way that it can be connected, on the one hand, to the output shaft of the step-by-step variable transmission via a clutch and, on the other hand, to the transmission input shaft via a clutch (referred to below as the “Fischer solution”). This arrangement is considered to be particularly favorable since it permits starter generator operation, recuperation and load shifting with boosting of the traction force.
This variant of the prior art has been used to form the preamble of claim 1.
An example of a drive train in which an electric machine is connected to a shaft of the step-by-step variable speed transmission is known from DE 199 31 770 A1.
Instead of a synchronization device, the clutch for the third gear speed is embodied as a friction clutch.
DE 199 60 621 A1 discloses a hybrid drive for vehicles, in which a step-by-step variable speed transmission is divided into two component transmissions. One of the component transmissions is connected to an electric machine. The other component transmission can optionally have a drive connection to an internal combustion engine and/or to the electric machine. The output shaft of the electric machine is connected to a shaft of the step-by-step variable speed transmission.
DE 295 02 906 U1 also discloses a hybrid drive wherein the rotor of an electric machine is rotatably mounted on the crankshaft of the internal combustion engine. The rotor can be connected to the crankshaft by means of a first separating clutch. The rotor can be connected to a transmission input shaft by means of a second separating clutch.
Finally, DE 101 33 695 A1 discloses a double clutch transmission which has two component transmissions. At least one of the transmissions can be connected to an electric machine.